Process analytic systems are used in a variety of industries to sense the quantity and/or quality of one or more analytical parameters of interest. One example of such an environment is the combustion process itself. Combustion generally consumes a quantity of oxygen and an organic compound and provides, ideally, carbon dioxide and water. In the real world, however, combustion is often not totally complete. This leaves a relatively small quantity of unused non-combusted material referred to hereinafter as “combustibles” and/or unused oxygen. There are certainly other environments in which knowledge of the concentration of combustibles and/or oxygen is desirable, and aspects of the present invention described herein are usable in such environments as well.
Many process analytic sensors use platinum and/or compounds thereof for sensing. Platinum provides a number of advantages in that it is generally highly robust in most analytic environments and provides temperature sensitivity. Temperature sensitivity means that generally, as the temperature of platinum metal changes, the resistance thereof will change in a predictable manner. Accordingly platinum is a frequently used and effective material in high temperature process analytic environments, and is widely used in both combustible sensors and oxygen sensors.
One potential drawback of platinum as a component of such sensors arises when sulfur-containing compounds are exposed to the sensor. Under reducing conditions, sulfur dioxide, for example, will react with combustibles present in a flue stream thereby forming gaseous sulfur in the following manner.SO2+2CO⇄S(g)+2CO2 
Gaseous sulfur subsequently reacts with platinum materials within the sensor forming volatile mixed valence platinum sulfides as described by G. Zwingmann and E. M. Wenzel, Reaction of Sulfur and Sulfur Containing Substances With Pt, Rd and Pt/Rd Alloys, METALL. 25 (1971) 1121. The reaction with sulfur can lead to evaporation of platinum within the sensor especially when it is disposed on ceramic such as in the case of analytic oxygen sensors and can lead to rapid electrode deterioration within the sensors.
With respect to prior art sensor electrodes, sulfur tolerance of composite electrodes has been taught. For example, U.S. Pat. No. 4,702,971 teaches a sulfur tolerant composite cermet electrode for solid oxide electrochemical cells. These electrodes can include an oxide selected from the group zirconium, yttrium, scandium, thorium, rare earth metals, and mixtures thereof. More reliable mixed conducting materials have been developed based on fluorite-type oxide ion conducting solid electrolytes, i.e. based on ceria, having considerably higher ionic and electronic conductivity. A description of such materials can be found in a paper by P. Shuk, M. Greenblatt and M. Croft, entitled “Hydrothermal Synthesis and Properties of the Mixed Conducting Ce1-xTbxO2-x/2 Solid Solutions. CHEM. MATER. 11 (1999) 473.
Providing process analytic electrochemical sensors that can withstand the high temperature environments of today's industrial demands while simultaneously resisting the effects of sulfur in sulfur-containing environments would be a vast improvement to the art since sulfur is present, to a greater or lesser degree in many environments. Additionally, analytical sensors which last longer in such environments necessarily reduce the amount of technician time required to maintain the process and may even potentially reduce overall operation cost of the combustion operation.